The genus Bordetella comprises four species Bordetella pertussis, Bordetella parapertussis, Bordetella bronchiseptica and Bordetella avium.
The bordetellae are Gram-negative coccobacilli responsible for respiratory infections. Bordetella pertussis and Bordetella parapertussis, agents of whooping cough, are strictly human pathogens. Bordetella bronchiseptica is pathogenic for various mammals, and more rarely for man, and, in distinction to B. pertussis and B. parapertussis, is capable of surviving outside the host. Bordetella avium is pathogenic only for birds.
Since the introduction of whooping cough vaccination in countries where vaccine cover is greater than 80%, it has been possible to observe a dramatic fall in morbidity and mortality. This fall is indeed attributable to vaccination since, in several countries (Great Britain, Sweden, Japan, etc.), deadly epidemics of whooping cough have taken place in the years following the cessation of vaccination.
The invention affords immunogenic compositions which can participate in the making of whooping cough vaccines, these compositions being at least partly of the "acellular" type and displaying an efficacy at least identical to that of the known vaccine.
The invention relates, on the one hand to vaccines which can be used in veterinary medicine, and on the other hand to vaccines which can be used in human medicine.
The whooping cough vaccine in current use is a cellular vaccine composed of heat-inactivated bacterial suspensions of B. pertussis (mixture of two strains differing in the expression of agglutinogens). This vaccine is generally used in combined form with purified diphtheria and tetanus fractions, the hemophilus and the inactivated polio viral component. Vaccination consists of three injections at one-month intervals from the age of two months and an injection at 18 months. No other booster injection is performed thereafter.
This vaccine is sometimes poorly tolerated, both locally and generally. It has, in particular, been blamed for giving rise to serious neurological complications of the acute encephalitis type; however, very recent studies appear to conclude that there is no statistical proof of a relationship between the cellular vaccine and the severe neurological complications (Griffiths AH. Vaccine 1989; 7:199-210).
It remains nonetheless true that the cellular vaccine is poorly tolerated and is responsible for reversible, but undesirable, effects. For these reasons, a new vaccine lacking these effects is desirable. In order to envisage the defining of a new vaccine, it appeared necessary to characterize certain factors involved in the virulence of the bacterium and, where appropriate, in the regulation of the virulence. When purified, each and every one of these different factors is a theoretical candidate for the making of a whooping cough vaccine termed "acellular" as opposed to the traditional vaccine. This new type of vaccine should provide, in addition to better tolerance, an efficacy at least equal to that of the traditional vaccine.
The factors involved in the virulence of B. pertussis have been identified as follows: whooping cough may be defined in broad outline by the association of an infectious syndrome, involving the adhesion of the bacteria to the target cells (ciliated cells of the respiratory apparatus), without invasion or dissemination in the host's body, and of a secondary toxin-induced syndrome including the local cytopathogenic effects which are elective for the ciliated respiratory epithelium (destruction and removal of ciliated cells, accumulation of mucus, inflammatory reaction) and systemic effects, the most obvious of which is hyperleucocytosis with hyperlymphocytosis.
As a result of recent techniques in molecular biology, a number of factors involved in the virulence of B. pertussis have been characterized and the regulation of their expression understood. These factors may be classified in two categories, those participating in the infectious syndrome (adhesins) and those playing a part in the toxin-induced syndrome (toxins).
The adhesins:
filamentous hemagglutinin or FHA is considered to play a major part in the adhesion of the bacterium to the ciliated epithelium (Locht C., Bertin P., Menozzi F. D. and Renaud G. Mol. Microbiol. 1993, 9:653-66). FHA is always expressed by the virulent strains and is secreted. Its structural gene has been cloned and sequenced (Relman D. et al., 1989, Proc. Natl. Acad. Sci. USA, 86:2637-2641). It codes for a protein of 360 kDa, but only a 220-kDa fragment can be purified. This protein binds to the glycoproteins of ciliated cells and possesses binding sites for the integrins of lymphocytes and macrophages. It has just been shown recently that FHA displays a homology with certain proteins of the host's endothelial cells (Tuomanen E., Prasad S. M., George J. S., Hoepelman A. I. M., Ibsen P., Heron I., and Starzyk R. M. 1993. Proc. Natl. Sci. USA. 90:7824-7828). PA1 The two agglutinogens or AGGs of B. pertussis enable strains to be classified in serotypes. Two AGGs have been characterized. These proteins are secreted and play a part in the adhesion of the bacterium to the epithelial cells (Mooi F. Van der Heide H. G. D., Ter Avest A. R., Welinder K. G., Livey I., Van der Zeijst B. A. M., and Gaastra, W. 1987. Microb. Pathog. 2:473-484). PA1 Pertactin or PRN is a protein of 93 kDa, but only a 69-kDa fragment can be purified. This protein possesses two binding sites for the integrins of macrophages and of lymphocytes (Charles I, Dougan G., Pickard D., Chattfield S. Smith M. Novotny P., Morissey P. and Fairweather N. F. 1989 Proc Natl Acad Sci. 86: 3554-3558). PA1 Pertussis toxin or PTX, a secreted type A-B toxin which, besides its cytopathogenic effects, participates in adhesion via its B subunit. The B oligomer is capable of binding to the receptors of ciliated cells, but not necessarily to the same receptors as those for FHA. The binding of PTX to leukocytes would appear to prevent their migration to the site of the inflammatory reaction. This binding appears to induce an increase in the number of functional integrin molecules to leukocytes, thereby promoting binding of the bacterium via FHA (Rozindski E., Burnette W. N., Jones T., Mar V., and Tuomanen E. 1993 J. Exp. Med. 178:917-924). PA1 Pertussis toxin or PTX is secreted and considered to be the major toxin of B. pertussis. Its A subunit possesses ADP-ribosyl transferase activity. After binding of the B portion of the toxin to the target cell, this A subunit is capable of entering the cell, of inactivating the regulatory G proteins and thus of causing interference with all cellular functions. It is this factor which appears to be responsible for the generalized biological effects observed during the disease, such as hyperlymphocytosis, hyperinsulinemia and sensitivity to histamine. PA1 Dermonecrotic toxin or DNT, which has not yet been well characterized, and tracheal cytotoxin or TCT, a secreted small glycoprotein of the muramyl peptide family, derived from the peptidoglycan of the bacterium, appear to act in concert to destroy the ciliated cells of the host's respiratory apparatus. TCT prevents, in addition, the regeneration of the respiratory epithelium (Luker K., Collier J. L., Kolodziej E. W., Marshall G. R., and Goldman W. E. 1993. Proc. Natl. Acad. Sci. USA. 90:2365-2369). PA1 Adenyl cyclase-hemolysin or Ac-Hly is a bifunctional protein possessing adenyl cyclase activity and hemolytic activity. It is secreted by the bacterium. Its structural gene has been cloned and sequenced (Glaser P. et al., 1988, Molec. Microb. 2, 19-20). This protein has been found to belong to the family of toxins termed "RTX" for "repeats in toxins", and displays homologies with the hemolysin of Escherichia coli and of Actinobacillus pleuropneumoniae and the leukotoxins of Pasteurella haemolytica and of Actinobacillus actinomycetemcomitans. This protein, like PTX, is capable of entering eukaryotic cells such as macrophages, of being activated by calmodulin, of synthesizing large amounts of cAMP and of interfering with the cellular functions (Coote J. 1992. FEMS Microbiol. Rev. 88:137-162).
The toxins:
Similarly, the factors involved in the virulence of B. parapertussis and B. bronchiseptica have been identified.
B. pertussis, B. parapertussis and B. bronchiseptica infections are indistinguishable from a clinical standpoint. These bacteria have more than 75% homology in respect of the DNA. They have been classified in species only on the basis of phenotypic and biochemical characters. B. paraertussis and B. bronchiseptica synthesize virulence factors functionally and immunologically very close to B. pertussis, with the exception of PTX.
A vaccine composed of bacterial suspensions of inactivated B. pertussis protects against a B. pertussis infection, but also against a B. parapertussis and a B. bronchiseptica infection in the mouse model. Although there are no epidemiological data on B. parapertussis infections in France, it should be noted that few strains of this species are isolated in this country, a country which has been vaccinated for 25 years with a "pertussis vaccine", whereas they are isolated in unvaccinated or poorly vaccinated countries.
Besides the presence of these different adhesins and toxins, the bordetellae are characterized by a regulation of the expression of the factors involved in their virulence. In other words, the bordetellae undergo phase variations and modulations.
The bordetellae, depending on their environment, may become "avirulent", that is to say incapable of inducing lethality, an inflammatory reaction and pulmonary lesions in the mouse model of respiratory infection. They undergo either a phase modulation or a phase variation. The phase variation is observed at a frequency ranging from 10.sup.-3 to 10.sup.-6 and is almost irreversible. It manifests itself in a cessation of the expression of the toxins and adhesins described above and in the expression of other factors which have not yet been well characterized (change of Phase I "virulent" bacteria to Phase IV "avirulent" bacteria). The Phase I and Phase IV bacteria have been described by Lacey B. 1960, J. Hyg. 58:57-93. The phase modulation, phenotypically similar to the phase variation, is completely reversible and manifests itself in a cessation of the synthesis of the adhesins and the toxins when there are environmental changes (composition of the culture medium, temperature, etc.).
The phase variation and phase modulation observed in Bordetella are under the control of two regulatory genes bvg A and bvg S (Arico B. et al., 1989, Proc. Natl. Acad. Sci USA, 86: 6671-6675).
The bvg S gene codes for a protein sensitive to external conditions. This protein modulates by phosphorylation the activity of the protein encoded by the bvgA gene, which is, on the one hand a positive activator of the transcription of the genes coding for the virulence factors (vag genes for "vir activated genes") mentioned above (Uhl M. A. and Miller J. 1994. Proc. Natl. Acad. Sci USA 91:1163-1167), and on the other hand a repressor of the transcription of certain genes (Beattie D. T. et al., J. of Bacteriology, Jan 93, p. 159-527). The genes whose expression is repressed are referred to as vrg genes for "vir repressed genes", and are still poorly characterized. It has, however, been shown that the vrg 6 gene of B. pertussis codes for a protein having a role in the peristance of the bacterium in the host (Beatties D. et al., 1992, Infect. Imm. 60:571-577). In B. bronchiseptica, two proteins encoded by the vrg genes have been characterized: they are proteins of the flagella type (Phase I B. bronchiseptica is an immobile bacterium which does not synthesize flagella but synthesizes adhesins and toxins, and Phase IV B. bronchiseptica is a mobile bacterium which synthesizes flagella).
In order to measure the virulence of the bacteria and to evaluate the local and general toxinic effects, a mouse model of respiratory infection has been developed (Guiso N. et al., 1991, Microb. Pathogen 11, 423-431). Using this mouse model, it has been possible to show that chemically or genetically inactivated PTX is a good immunogen. This anatoxin has a protective activity against lethal B. pertussis infections, but does not appear to induce the synthesis of effective antibodies against the persistance of the bacterium (Khelef. N. Danve B. Quentin-Millet M. J. and Guiso N. 1993 Infect Immun. 64:486-490).
These results relating to the virulence of bordetellae and to the regulation of this virulence show that whooping cough is a multifactorial disease and that the vaccine must not only protect against lethal infections but also against the persistance of the bacterium. Similar conclusions apply to the infections due to B. parapertussis or to B. bronchiseptica.
Attempts to develop an acellular vaccine from the isolated components of the adhesin or toxin family have been performed. Thus, acellular compositions containing either purified B. pertussis toxin (PTX) or this toxin combined with purified filamentous hemagglutinin (FHA) have been prepared.
The first trials of tolerance of these acellular compositions (PTX or PTX-FIA) in man show a marked decrease in both local complications (pain, swelling) and general complications (fever, convulsions, etc.) in comparison with the traditional cellular vaccine (Edward K., J. Infect. Dis. 1993, 168, 15-20).
These new preparations (PTX or PTX-FHA) have good immunogenicity and induce a high level of antibodies. However, the test for vaccinal antibodies is an imperfect method, since seroconversion is not synonymous with protection against the disease, and no demonstration has been made of the protective character of the antibodies obtained, or of the possible level of protection.
The results of clinical trials of different cellular and acellular vaccines have been published (International Symposium on Pertussis Vaccine trials, Rome 30.10.95-1.11.95). These results show that not all the cellular vaccines are equivalent, some are very effective and induce few side effects and others are of very low efficacy and induce greater side effects.
The published results show that the acellular vaccines tested, monovalent (PTX), bivalent (PTX, FHA), trivalent (PTX, FHA, PRN) or pentavalent (PTX, FHA, PRN, AGG2, AGG3) induce very few side effects, are all immunogenic and all have an efficacy against the disease (according to WHO definition) which is greater than or equal to 70%. However, the efficacy of an acellular PTX-FHA vaccine is always less, irrespective of the particular definition which may be used, than that of an effective cellular vaccine.